1. Field of the Invention
The present invention concerns a method for implementation of a magnetic resonance examination (MR examination) with an MR apparatus, of the type wherein MR signals with an MR frequency are acquired from an examination region with an antenna array after a radio-frequency excitation (RF excitation). The invention furthermore concerns a magnetic resonance apparatus and an antenna array for acquisition of magnetic resonance signals.
2. Description of the Prior Art
MR examinations enable spatially-resolved imaging of an examination region. For reasons of the achievable signal-to-noise ratio, in MR apparatuses it is advantageous to use local or surface coils for acquisition of MR signals from only a limited small region of the subject. Such coils are normally connected with an evaluation device via cables. The cables and the necessary plug contacts represent a limitation to the user-friendliness and the reliability of the surface coil and the corresponding signal transmission. It is known to transfer magnetic resonance signals from the coil arrangement to the evaluation unit via an optical cable.
It is also known to convert an acquired magnetic resonance signal into another frequency and to wirelessly transmit it to the evaluation device. Such a linear frequency conversion with transistor or diode mixers is known, for example, from DE 41 26 537 A1. The receiver for the frequency-converted signal is located only a few meters from the site of the evaluation unit. If an array of coils that supply MR signals independently of one another is used for the acquisition of MR signals from different body segments, these MR signals are then converted into different frequencies. In the frequency conversion, the MR signal is mixed with a mixer signal having a constant frequency in a mixing stage.
Furthermore, a linear frequency conversion with parametric amplification is known from DE 102 19 749 A1. In the transmission method based thereupon, an MR signal is acquired by an MR reception antenna at an MR frequency and is supplied to a non-linear reactance. Auxiliary (supplementary) energy is supplied by an auxiliary antenna with at least one auxiliary frequency. One part of the auxiliary energy is acquired by the auxiliary reception antenna and supplied with the auxiliary frequency to the non-linear reactance and there is mixed together with the MR signal to form a mix signal with a mix frequency. The mix signal is supplied to a mix signal transmission antenna and is emitted thereby. It is subsequently acquired by a mix signal reception antenna and supplied to the evaluation device.
When an additive signal at an additive frequency, that is equal to the difference of the auxiliary frequency and the magnetic resonance frequency is generated by the non-linear reactance and the additive signal is supplied to an absorption circuit resonant at the additive frequency, a higher radiated energy results at the mix frequency. The absorption circuit is connected in parallel with, for example, the non-linear reactance. The energy emitted by the mix signal transmission antenna becomes greater the further that the auxiliary frequency deviates from the MR frequency. Such an energy supply enables a locally-acquired MR signal to be wirelessly transmitted to a control and evaluation device in a simple manner. A transfer of MR signals that are acquired by a number of coils of an array is also possible with the method according to DE 102 19 749 A1. For this purpose, the auxiliary frequencies and the mix frequencies resulting from the auxiliary frequencies and the MR signals are selected such that the individual auxiliary and mix signal frequencies do not mutually influence one another. It is thereby even possible to emit an auxiliary signal via a single broadband auxiliary transmission antenna, with the auxiliary signal containing a number of auxiliary frequencies that differ from one another. The auxiliary signal can be acquired by a single broadband auxiliary reception antenna and be supplied to the mixer arrangements via input filter circuits.
A transmission method for an analog magnetic resonance signal by means of frequency modulation is known from DE 101 48 462 C1. Method for digital transmission with time, frequency or code multiplexing are known from telecommunications.